1. Field of the Invention
The present invention relates to a process for producing a golf ball having a cover formed from a thermoplastic polyurethane material; and more particularly to a process for producing a golf ball having a cover formed from a thermoplastic polyurethane material which can be recycled for molding, which exhibits high restitution, and which exhibits excellent scuff resistance.
2. Description of the Related Art
In recent years, polyurethane materials have become of interest as materials for forming a golf ball cover. Polyurethane materials are classified into thermosetting polyurethane materials and thermoplastic polyurethane materials, and a process for forming a thermosetting polyurethane material into a product differs from a process for forming a thermoplastic polyurethane material into a product. A thermosetting polyurethane material can be formed into a product through the following procedure: a urethane prepolymer having an isocyanate end group and a curing agent such as polyol or polyamine, which serve as liquid raw materials, are mixed under heating; and the resultant mixture is fed directly to a mold and then heated, to thereby allow urethane curing reaction to proceed.
Many studies have heretofore focused on golf balls formed from thermosetting polyurethane materials. For example, U.S. Pat. Nos. 5,334,673, 6,117,024, and 6,190,268 disclose such golf balls. Meanwhile, U.S. Pat. Nos. 5,006,297, 5,733,428, 5,888,437, 5,897,884, and 5,947,843 disclose forming methods of thermosetting polyurethane materials.
Since a thermosetting polyurethane material exhibits no thermoplasticity, the material and a product formed from the material cannot be recycled. In addition, when a thermosetting polyurethane material is employed for forming a specific product such as a golf ball cover (i.e., a product which covers a core), efficient production of the product is not attained, since the heating-curing step and the cooling step of the material requires long time, and high reactivity and instability of the material make control of the molding time very difficult.
In the case where a thermoplastic polyurethane material is formed into a molded product, the product is not directly obtained through reaction of raw materials, but is formed from a linear polyurethane materialxe2x80x94an intermediatexe2x80x94which has been synthesized by employment of raw materials and a synthesis method, the raw materials and the method differing from those employed in the case of the aforementioned thermosetting polyurethane material. Such a linear polyurethane material exhibits thermoplasticity, and is cured through cooling. Therefore, such a polyurethane material can be molded by use of an injection molding machine. Injection molding of a thermoplastic polyurethane material is a technique suitable for forming a golf ball cover, since the molding time of a thermoplastic polyurethane material is much shorter than that of a thermosetting polyurethane material, and a thermoplastic polyurethane material is suitable for precise molding.
Meanwhile, a thermoplastic polyurethane material can be recycled, and is thus environmentally friendly. U.S. Pat. Nos. 3,395,109, 4,248,432, and 4,442,282 disclose golf balls formed from thermoplastic polyurethane materials.
However, when a golf ball cover is formed from a conventional thermoplastic polyurethane material, the resultant golf ball is not satisfactory in terms of feeling on impact, controllability, restitution, and scuff resistance upon being hit with an iron.
In order to solve such a problem, Japanese Patent Application Laid-Open (kokai) No. 9-271538 discloses a golf ball cover formed from a thermoplastic polyurethane material exhibiting high restitution. However, the disclosed golf ball cover is not satisfactory in terms of scuff resistance upon being hit with an iron.
Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses a golf ball cover exhibiting relatively excellent scuff resistance upon being hit with an iron, which predominantly contains a reaction product formed from a thermoplastic polyurethane material and an isocyanate compound. When the cover is formed, an isocyanate compound such as a diisocyanate or a block isocyanate dimer, serving as an additive, is added to a thermoplastic polyurethane material in the course of heating, melting, and mixing by use of an extruder, or in the course of injection molding, to thereby allow reaction to proceed.
However, in the case of molding of the cover disclosed in Japanese Patent Application Laid-Open (kokai) No. 11-178949, since an isocyanate compound must be handled with great care due to its inactivation by moisture, obtaining a stable reaction product is difficult. Meanwhile, a block isocyanate exhibiting moisture resistance is not suitable for forming the cover, since a blocking agent issues a strong odor when the isocyanate is thermally dissociated. When an isocyanate compound assumes the form of powder or solution, control of the amount of the compound which is added to a thermoplastic polyurethane material is difficult, and therefore cover properties cannot be controlled adequately. In addition, since the thermoplastic polyurethane material differs in melting point and melt viscosity from the isocyanate compound, thorough and satisfactory kneading thereof may fail to be attained in a molding apparatus. Therefore, in the technique disclosed in the above publication, the effect of moisture on a cover material and the amount of an additive is not satisfactorily controlled, resulting in failure to produce a golf ball cover which is satisfactory in terms of improvement of scuff resistance.
Japanese Patent Application Laid-Open (kokai) No. 11-178949 discloses an aliphatic isocyanate-based thermoplastic polyurethane material to be used as a desirable thermoplastic polyurethane material. However, since the thermoplastic polyurethane material is highly reactive with an isocyanate and its reaction is difficult to control, the polyurethane material involves the following problems: gelation easily occurs before injection molding, and sufficient plasticity cannot be maintained; gelation may occur during molding of a cover; and the polyurethane material cannot be recycled, due to gelation. Because of such problems, the thermoplastic polyurethane material is difficult to use in practice.
Japanese Patent Publication (kokoku) No. 58-2063 (U.S. Pat. No. 4,347,338) discloses a process for producing a thermosetting polyurethane product, in which a compound having two or more isocyanate groups is mixed with a thermoplastic resin which is non-reactive with an isocyanate group, the resultant mixture is incorporated into a thermoplastic polyurethane material, and the resultant material is subjected to molding by use of a molding machine. However, the purpose of the technique disclosed in the above publication is to improve the polyurethane product only in terms of solvent resistance and durability against continuous, repeating friction, and the publication does not disclose use of the aforementioned forming material as a material of a golf ball cover. There still exists demand for a golf ball cover material which can provide a golf ball with various necessary properties, such as restitution, total distance, spin performance, controllability, feeling on impact, scuff resistance, cut resistance, and discoloration resistance. Moreover, there exists demand for a process for producing a golf ball utilizing properties of such a material.
In view of the foregoing, an object of the present invention is to provide a process for producing a golf ball having a cover formed from a thermoplastic polyurethane material which can be recycled (i.e., remolded), which exhibits high restitution, and which exhibits excellent scuff resistance.
In order to achieve the above object, the present invention provides the following production processes for a golf ball.
(1) A process for producing a golf ball comprising a core and a cover therefor, which comprises preparing a cover-forming material (C) by incorporating the following component (B) into the following component (A); and subjecting the cover-forming material (C) to injection molding, to thereby form a cover around the core:
(A) a thermoplastic polyurethane material, and
(B) an isocyanate mixture in which an isocyanate compound (b-1) having at least two isocyanate groups serving as functional groups in the molecule is dispersed in a thermoplastic resin (b-2) which is substantially non-reactive with the isocyanate groups.
(2) A process for producing a golf ball according to (1), wherein the thermoplastic polyurethane material (A) is synthesized from a polyether polyol and an aromatic diisocyanate.
(3) A process for producing a golf ball according to (2), wherein the polyether polyol is a polytetramethylene glycol having an average molecular weight of at least 2,000.
(4) A process for producing a golf ball according to (2) or (3), wherein the aromatic diisocyanate is 4,4xe2x80x2-diphenylmethane diisocyanate.
(5) A process for producing a golf ball according to any one of (1) through (4), wherein the isocyanate compound (b-1) is 4,4xe2x80x2-diphenylmethane diisocyanate.
(6) A process for producing a golf ball according to any one of (1) through (5), wherein the thermoplastic resin (b-2) is a polyester elastomer.
(7) A process for producing a golf ball according to any one of (1) through (6), wherein, in the isocyanate mixture (B), the ratio by weight of the thermoplastic resin (b-2) to the isocyanate compound (b-1) is 100:5 to 100:100.
(8) A process for producing a golf ball according to any one of (1) through (7), wherein, in the cover-forming material (C), the ratio by weight of the thermoplastic polyurethane material (A) to the isocyanate mixture (B) is 100:1 to 100:40.
(9) A process for producing a golf ball according to any one of (1) through (8), wherein the cover-forming material has a surface hardness of 40 to 80 as measured by use of a D-type durometer after having undergone injection molding, and a restitution elastic modulus of at least 45%.
(10) A process for producing a golf ball according to any one of (1) through (9), wherein the cover-forming material can be recycled for molding after the material has undergone injection molding.
The present invention will next be described in more detail. Firstly, components (A) to (C) will be described.
(A) Thermoplastic Polyurethane Material
The thermoplastic polyurethane material includes soft segments formed of a polymeric polyol (polymeric glycol), a chain extender constituting hard segments, and a diisocyanate. No particular limitation is imposed on the polymeric polyol serving as a raw material, and the polymeric polyol may be any one selected from polymeric polyols which are conventionally employed in the technical field related to thermoplastic polyurethane materials. Examples of the polymeric polyol include polyester polyols and polyether polyols. Of these, polyether polyols are more preferred to polyester polyols, since a thermoplastic polyurethane material having high restitution elastic modulus and exhibiting excellent low-temperature properties can be synthesized. Examples of the polyether polyols include polytetramethylene glycol and polypropylene glycol. From the viewpoints of restitution elastic modulus and low-temperature properties, polytetramethylene glycol is particularly preferred. The average molecular weight of the polymeric polyol is preferably 1,000 to 5,000. The average molecular weight is more preferably 2,000 to 4,000, in order to synthesize a thermoplastic polyurethane material having high restitution elastic modulus.
Any chain extender which is conventionally employed in the technical field related to thermoplastic polyurethane materials is preferably used. Examples of the chain extender include, but are not limited to, 1,4-butylene glycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol, and 2,2-dimethyl-1,3-propanediol. The average molecular weight of the chain extender is preferably 20 to 15,000.
Any diisocyanate which is conventionally employed in the technical field related to thermoplastic polyurethane materials is preferably used. Examples of the diisocyanate include, but are not limited to, aromatic diisocyanates such as 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. Some diisocyanates involve difficulty in controlling cross-linking reaction during injection molding. In the present invention, 4,4xe2x80x2-diphenylmethane diisocyanate, which is an aromatic diisocyanate, is most preferred, in consideration of stability in reaction with the below-described isocyanate mixture (B).
In the present invention, a thermoplastic polyurethane material synthesized from a polyether polyol and an aromatic diisocyanate is most preferred; the polyether polyol is polytetramethylene glycol having an average molecular weight of at least 2,000, and the aromatic diisocyanate is 4,4xe2x80x2-diphenylmethane diisocyanate.
Preferred examples of the thermoplastic polyurethane material containing the aforementioned materials include commercially available polyurethane materials, such as Pandex T-8290, T-8295, and T-8260 (products of DIC Bayer Polymer Ltd.), and Resamine 2593 and 2597 (products of Dainichiseika Color and Chemicals Mfg. Co., Ltd.).
(B) Isocyanate Mixture
The isocyanate mixture (B) is obtained by dispersing the isocyanate compound (b-1) having at least two isocyanate groups serving as functional groups in the molecule in the thermoplastic resin (b-2) which is substantially non-reactive with the isocyanate groups. The aforementioned isocyanate compound (b-1) is preferably an isocyanate compound which is conventionally employed in the technical field related to thermoplastic polyurethane materials. Examples of the isocyanate compound include, but are not limited to, aromatic diisocyanates such as 4,4xe2x80x2-diphenylmethane diisocyanate, 2,4-toluene diisocyanate, and 2,6-toluene diisocyanate; and aliphatic diisocyanates such as hexamethylene diisocyanate. Of these, 4,4xe2x80x2-diphenylmethane diisocyanate is most preferred, in consideration of reactivity and operational safety.
The aforementioned thermoplastic resin (b-2) is preferably a resin having low water-absorbability and high compatibility with the thermoplastic polyurethane material. Examples of the resin include polystyrene resins, polyvinyl chloride resins, ABS resins, polycarbonate resins, and polyester elastomers (e.g., polyether-ester block copolymers and polyester-ester block copolymers). Of these, in consideration of restitution elasticity and strength, polyether-ester block copolymers are particularly preferred.
In the isocyanate mixture (B), the ratio by weight of the thermoplastic resin (b-2) to the isocyanate compound (b-1) is preferably 100:5 to 100:100, more preferably 100:10 to 100:40. When the ratio of the isocyanate compound (b-1) to the thermoplastic resin (b-2) is excessively low, a large amount of the isocyanate mixture (B) must be added to the thermoplastic polyurethane material (A), in order to achieve a successful cross-linking reaction between the isocyanate compound (b-1) and the thermoplastic polyurethane material (A). As a result, the thermoplastic resin (b-2) greatly affects the thermoplastic polyurethane material (A), resulting in unsatisfactory properties of the cover-forming material (C). In contrast, when the ratio of the isocyanate compound (b-1) to the thermoplastic resin (b-2) is excessively high, thorough and satisfactory kneading of the isocyanate compound (b-1) into the thermoplastic resin (b-2) is not attained, and thus preparation of the isocyanate mixture (B) becomes difficult.
The isocyanate mixture (B) can be obtained through, for example, the following procedure: the isocyanate compound (b-1) is incorporated into the thermoplastic resin (b-2), and the resultant mixture is completely kneaded by use of a mixing roll or a banbury mixer at 130 to 250xc2x0 C., followed by pelletization or pulverization after cooling. Preferred examples of the isocyanate mixture (B) include commercially available isocyanate mixtures such as Crossnate EM30 (product of Dainichiseika Color and Chemicals Mfg. Co., Ltd.).
(C) Cover-Forming Material
The cover-forming material (C) is obtained by incorporating the isocyanate mixture (B) into the thermoplastic polyurethane material (A). In the cover-forming material (C), the ratio by weight of the thermoplastic polyurethane material (A) to the isocyanate mixture (B) is preferably 100:1 to 100:100, more preferably 100:5 to 100:50, much more preferably 100:10 to 100:30. When the ratio of the isocyanate mixture (B) to the thermoplastic polyurethane material (A) is excessively low, the isocyanate mixture (B) exerts insufficient cross-linking effect, whereas when the ratio is excessively high, unreacted isocyanate imparts a color to the resultant cover-forming material.
In the present invention, the cover-forming material (C) may contain other components in addition to the aforementioned components (A) and (B). Examples of such xe2x80x9cother componentsxe2x80x9d include thermoplastic polymer materials other than the thermoplastic polyurethane material, such as polyester elastomer, polyamide elastomer, ionomer resin, styrene block elastomer, polyethylene, and nylon resin. In this case, the incorporation amount of thermoplastic polymer materials other than the thermoplastic polyurethane material is 0 to 100 parts by weight, preferably 10 to 75 parts by weight, more preferably 10 to 50 parts by weight, on the basis of 100 parts by weight of the thermoplastic polyurethane material which serves as an essential component. The incorporation amount is appropriately determined in accordance with various purposes, including regulation of the hardness of the cover-forming material and improvement of the restitution, fluidity, and adhesion of the cover-forming material. If desired, the cover-forming material (C) may further contain various additives, such as pigments, dispersants, antioxidants, light-resistant stabilizers, UV absorbers, and release agents.
In the present invention, a cover can be formed through, for example, the following procedure: the isocyanate mixture (B) is added to the thermoplastic polyurethane material (A) and then dry-mixed, and a cover is formed from the resultant mixture around a core by use of an injection molding apparatus. The molding temperature varies with the type of the thermoplastic polyurethane material (A), but is typically 150 to 250xc2x0 C.
In the resultant golf ball cover, reaction or cross-linking is thought to proceed as follows: an isocyanate group is reacted with a residual OH group of the thermoplastic polyurethane material, to thereby form a urethane bond; or an isocyanate group is added to a urethane group of the thermoplastic polyurethane material, to thereby form an allophanate or biuret cross-linking structure. In this case, although cross-linking proceeds insufficiently immediately after injection molding of the cover-forming material (C), cross-linking proceeds through annealing after injection molding, and the resultant golf ball cover is endowed with useful properties. As used herein, the term xe2x80x9cannealingxe2x80x9d refers to aging through heating at a certain temperature for a predetermined period of time, or aging at room temperature for a predetermined period of time.
In the production process for a golf ball of the present invention, the surface hardness of the cover-forming material after having undergone injection molding is preferably 40 to 80, more preferably 43 to 60, much more preferably 45 to 55, as measured by use of a D-type durometer in accordance with JIS-K6253. When the surface hardness of the cover-forming material is excessively low, the resultant golf ball tends to produce excessive back-spin upon being hit with an iron; i.e., controllability of the golf ball is impaired. In contrast, when the surface hardness of the cover-forming material is excessively high, the resultant golf ball tends to produce insufficient back-spin upon being hit with an iron; i.e., controllability of the golf ball is lowered, and feeling on impact is impaired.
In the production process for a golf ball of the present invention, the restitution elastic modulus of the cover-forming material after injection molding is preferably at least 45%, more preferably 45 to 85%, further preferably 50 to 80%, much more preferably 50 to 60%, as specified by JIS-K7311. Since the thermoplastic polyurethane material does not exhibit high restitution, preferably, the restitution elastic modulus is strictly selected. When the restitution elastic modulus of the cover-forming material is excessively low, the total distance of the golf ball is considerably lowered. In contrast, when the restitution elastic modulus of the cover-forming material is excessively high, the initial velocity of the golf ball becomes excessively high when being shot or putted (i.e., when controllability of the golf is required within the range of a total distance of 100 yards or less), and the golf ball may fail to meet a golfer""s demand.
No particular limitation is imposed on the core employed in the golf ball of the present invention, and any type of cores that are usually employed can be employed. Examples of the core which may be employed include a solid core for a two-piece ball, a solid core having a plurality of vulcanized rubber layers, a solid core having a plurality of resin layers, and a thread-wound core having a thread rubber layer. No particular limitation is imposed on the outer diameter, weight, hardness, and material of the core. The thickness of the golf ball cover of the present invention preferably falls within a range of 0.1 to 5.0 mm. The cover may have a multi-layer structure, so long as the overall thickness of the cover falls within the above range.
The golf ball of the present invention is formed so as to have a diameter and a weight as specified under the Rules of Golf approved by RandA. Typically, the diameter is at least 42.67 mm, and the weight is 45.93 g or less. The diameter is preferably 42.67 to 42.9 mm. The deformation amount of the golf ball under application of a load of 980 N (100 kg) is preferably 2.0 to 4.0 mm, more preferably 2.2 to 3.8 mm.